This invention relates to a method of cold roll forming using a plurality of cold forming roll stands for producing cold roll formed shape strip having a predetermined curvature longitudinally thereof, and more particularly to a method of cold roll forming for producing a cold roll formed shape strip of special cross sectional shape curved longitudinally with a predetermined curvature and having its edge portions on the concave side of the curvature and its web on the convex side thereof.
In subjecting metal strip to cold roll forming to gradually give a predetermined shape to the metal strip by passing it through a plurality of cold forming roll stands, it has hitherto been usual practice to use a straightener to straighten the metal strip longitudinally. However, it has hitherto been customary to utilize press forming in producing curved products having a predetermined curvature longitudinally thereof. Press forming has been low in productivity, very low in efficiency in forming products of large length, and expensive. Also, in producing curved products by press forming, there has been raised the problem of developing galling on the surface of the product and thereby reducing its quality. Therefore, it has hitherto been earnestly desired to establish a method of cold roll forming capable of producing, with a high degree of efficiency, roll formed products, such as bumpers for automotive vehicles, which are produced on mass production basis. Lately, some of those skilled in the art made an attempt to form bumpers by cold roll forming as bumpers for automotive vehicles. In such a system fourteen stands of cold forming roll stands were used which have the same height of the point of contact between the metal strip to be roll formed. Rolls of each cold forming roll stand carried out longitudinally straight shaping, then curving was carried out longitudinally by four stands of cold forming roll stands. The radius of curvature of the roll formed product was fourteen meters. The twist of the roll formed product was reformed and the roll formed product was cut to a predetermined length.
However, a problem has been encountered with regard to buckling of rise portions of the roll formed product, therefore, straight shaping was carried out similarly by a previous fourteen stands of cold forming roll stands, then curving was carried out longitudinally by a combination of shoes and bending rolls, and then twist of the product was reformed and the radius of the curvature of the product was fourteen meters. Thus, a solution has been achieved with regard to preventing buckling to some extent, but in the curved product produced as explained above, a problem has been raised that galling occurred in bending corner portions of sectional shape of the curved product and galling occurred to the curved product by roll forming and shoe forming, further, the radius of the curvature was large, that is, fourteen meters.
As a result, it has not been established yet that the method of cold roll forming can form efficiently in a stable manner, cold roll formed strip of high quality which is free from surface defect, edge wave, buckling and deformation and has a small longitudinal curvature of the product. As mentioned above, difficulties have been encountered in producing, by cold roll forming, shape strip of light weight having a special cross sectional shape with lips in such a manner that the shape strip has a longitudinal predetermined curvature. In particular, a difficulty has been experienced in curving metal strip longitudinally to give a predetermined curvature thereto in such a manner that the edge portions of the product are on the concave side of the curvature and the web of the product is on the convex side of the curvature.
More specifically, in producing shape strip by using ordinary upper and lower rolls, forming of shape strip starts with the rise portions of the cross sectional shape and is successively effected until the lips are shaped. As a result, the lips are subjected to extensive elongation and the roll formed shape strip has a longitudinal curvature with the lips on the convex side of the curvature. Therefore, it has been necessary to take some measures to effect curving of the shape strip in a direction opposite to the aforesaid direction. When metal strip of small thickness and large width is cold roll formed into shape strip of light weight or a deck plate, a problem has been encountered with regard to irregular elongation and buckling of edge portions. This problem has been serious when metal strip is formed of stainless steel or other material of high tendency to spring back or when the metal strip has a large width and a small thickness and the shape to be given to the strip is complex. Generally, when a product has a circular cross sectional shape, such as a pipe, it has been believed impossible to obtain such a product by cold roll forming if the ratio T/D (wherein D is the diameter of the cross section of shaped product and T is the thickness thereof) is below 0.01. Thus, difficulties are experienced when roll formed product has a high rise relative to its width, is small in thickness, and is complex in cross sectional shape. Imbalance between the tensile and compressive deformation stresses acting on the side edge portions of strip and the deformation stresses acting on the widthwise central bottom portion of the strip would cause buckling of the edge portions of the formed product and result in forcible shaping of the strip. When this is the case, seizing and galling or deforming would occur in the interface between the rolls and rolled product, Thus, it would be impossible to produce a desired roll formed product. To inhibit the occurrence of buckling or elongation of the edge portions of strip, so-called downhill cold roll forming may be used in which the height of the point of contact between strip to be roll formed and the rolls of each cold forming roll stand is reduced from one stand to another as roll forming progresses, so as to suppress elongation of the edge portions and achieve balance in deformation of the roll formed product. There is also available so-called uphill cold roll forming in which the height of the point of contact between strip to be roll formed and the rolls of each cold forming roll stand is increased from one stand to another as the operation progresses, so as to effect roll forming by gradually imparting elongation deformation to marginal portions of the strip.